91 300CE M104, Coolant Level Light is On

[pantherpaw]

My 91 300CE (W124 with M104 engine) runs great, but the temperature is getting up a little higher than I like to see it when I'm in stop and go traffic. Sitting in traffic with, no A/C running, temp= 90 ~ 100 deg. C; which I assume is OK but higher than I like it to be. Sitting in traffic with A/C running, temp goes up over the 100 deg. C mark, but not all the way up to the RED MARK.

The Coolant Level Light comes on at times when hot, but goes back off again when I'm at cooler temps at cruising speeds.

I checked the level, and the coolant reservoir is full...What else could be wrong, or any advice would help greatly...